26 THE SYNTHESIS OF CARBOHYDRATES 



and Escombe,* the first to attempt the drawing of an energy 

 balance sheet of the leaf, concluded that Polygonum Weyrichii 

 used from -42 to I -66 per cent of the available radiant energy 

 for carbon assimilation, figures based partly on observation 

 and partly on calculation : Puriewitsch,-|- on the other hand, 

 found that Polygonum Sacchalinense similarly employed from 

 2*5 to 7*7 per cent of the radiant energy. 



Under no natural conditions is the full radiant energy made 

 use of by plants: the photosynthetic value of the noontide 

 sunshine at the summer solstice in these latitudes lies, according 

 to Blackman and MatthaeiJ between '04 and '05 grams of 

 carbon dioxide per 50 sq. cm. of leaf surface per hour ; the 

 highest assimilation actually measured by these workers was 

 0290 grams. 



The general statement that carbon assimilation varies with 

 the intensity of the illumination is true only when light is the 

 limiting factor ; other factors, particularly temperature, are 

 intimately associated in the process in nature. For this reason 

 it is impossible to consider the effect of one condition to the 

 exclusion of the other factors, a fact well demonstrated by 

 Blackman and Matthaei. In one of their experiments, an 

 abstract of which is given below, the leaves of Helianthus 

 tuberosus were surrounded by an atmosphere containing on the 

 average 4 per cent carbon dioxide and the light throughout 

 was diffused and of varying intensity. 



It will be seen from observations 2 to 4 and 6 and 7 that 

 the assimilation was remarkably uniform under conditions of 



* Brown and Escombe: " Proc. Roy. Soc.," Lond., B. 1905, 76, 29. 

 t Puriewitsch : " Jahrb. wiss. Bot.," 1914, 53, 210. 

 % Blackman and Matthaei : " Proc. Roy. Soc.," Lond., B. 1905, 76, 402. 

 Ibid. 



